1. Field of the Invention
The present invention relates to a hollow power transmission shaft coupled to a joint such as a constant velocity universal joint, and to a method of manufacturing the hollow power transmission shaft. The hollow power transmission shaft according to the present invention is applicable, for example, to a drive shaft or a propeller shaft which is a component of a power transmission system of an automobile.
2. Description of the Related Art
For example, in a power transmission system of an automobile, a power transmission shaft for transmitting power from a reduction gear (differential) to driving wheels is often referred to as a drive shaft. Especially in a drive shaft used for a front-engine front-drive vehicle, a large working angle and constancy of velocity are required in steering front wheels, and a function of absorbing axial displacement is required in relation to a suspension system. In many cases, therefore, there has been adopted a mechanism structured such that the drive shaft is coupled at one end thereof to a reduction gear side via a slidable constant velocity universal joint such as a double offset-type constant velocity universal joint or a tripod-type constant velocity universal joint, and at the other end thereof to a driving wheel side via a fixed constant velocity universal joint such as a Birfield-type constant velocity universal joint (which is also referred to as Rzeppa joint).
Presently as well as conventionally, a solid shaft has been often used as the drive shaft as mentioned above. Recently, however, there has been a growing demand for the adoption of a hollow shaft as the drive shaft with a view to saving weight of the automobile, achieving functional improvements through an increase in rigidity of the drive shaft, enhancing tranquility within a cabin of the automobile through optimization of tuning of a flexural primary natural frequency, and so on.
Examples of the hollow power transmission shaft applied to the drive shaft or the like are known from, for example, JP 2002-349538 A, JP 2002-356742 A, and JP 2003-90325 A.
In JP 2002-349538 A, an inner peripheral surface of a hollow shaft is subjected to a thermosetting treatment substantially over an entire axial range thereof. The thermosetting treatment is performed for an entire depth region extending from an outer peripheral surface of the hollow shaft to the inner peripheral surface thereof through, for example, induction hardening and tempering from the outer peripheral surface side (see paragraph 0012 in JP 2002-349538 A)
In JP 2002-356742 A, a thermosetting treatment is performed for an entire depth region extending from an outer peripheral surface of a hollow shaft to an inner peripheral surface thereof substantially over an entire axial range thereof through, for example, induction hardening and tempering (see paragraph 0012 in JP 2002-356742 A).
In JP 2003-90325 A, a hollow shaft is subjected to induction hardening at a hardening ratio of 0.7 to 0.9 in order to make a static strength and torsional fatigue strength of the hollow shaft equal to or higher than those of a solid shaft.
In the hollow shaft of this type, a spline for coupling the hollow shaft to a constant velocity universal joint or the like maybe formed at an end of the hollow shaft. So-called press working, namely, a process of axially press-fitting the end of the shaft into a dice having an inner periphery provided with a spline molding portion is known as a method of molding the spline (e.g., see JP 2003-094141 A). Press working is more advantageous than component rolling in that thin-walled articles can be handled as well.
A hollow power transmission shaft applied to a drive shaft or the like is manufactured by, for example, drawing a pipe material to mold a hollow shaft material having a large-diameter portion along an axially central portion thereof and small-diameter portions along axially opposite a lateral portion thereof, respectively, machining the hollow shaft material as desired according to need, and then heat-treating the hollow shaft material (e.g., see JP 11-101259 A and JP 2001-208037 A).
In the hollow power transmission shaft of this type, a sealing plug is fitted onto an inner periphery of each end of a hollow portion thereof so as to prevent a lubricant (grease) enclosed in the constant velocity universal joint from entering the hollow portion. In some cases, a metal plug is used as the sealing plug. However, the inner periphery of the end needs to be finished through shaving in order to control a press-fit margin and press-fit position with respect to the hollow portion. Consequently, a problem of higher working cost is caused. For this reason, there have been proposed a sealing plug formed of a rubber material such as chloroprene rubber (CR) or nitrile rubber (NBR) (JP 06-281010 A) and a sealing plug formed of elastomer (JP 09-68233 A).
In general, in order to achieve an increase in rigidity and a reduction in weight, the hollow power transmission shaft of this type has, along an axially central portion thereof, a large-diameter portion with a relatively small thickness, and has, along an axially opposite lateral portion thereof, small-diameter portions with a relatively large thickness so as to ensure a certain strength. Thus, the hollow power transmission shaft of this type has a difference in thickness in the axial direction thereof. Therefore, hardening conditions cannot be set with ease, and stable quality cannot be ensured by a heat treatment in some cases. That is, if the hardening conditions are set in accordance with the large-diameter portion with the relatively small thickness, there are some cases in which a depth of a hardened layer becomes insufficient in the small-diameter portions with the relatively large thickness, so a desired strength cannot be achieved. On the other hand, if the hardening conditions are set in accordance with the small-diameter portions with the relatively large thickness, there are some cases in which the large-diameter portion with the relatively small thickness is overheated, and a texture thereof becomes coarse to an extent of causing a decrease in strength after hardening.
In the hollow power transmission shaft of this type, for example, the large-diameter portion and the small-diameter portions may be made different in hardening ratio (ratio of the depth of the hardened layer to a wall thickness thereof) for the purpose of enhancing the balance of strength or the like. In a conventional manufacturing method, however, the same inconvenience as described above may occur.
In spline molding based on the conventional press working, the material plastically flows in a counter press-fit direction while an end of the shaft is axially press-fitted into a dice. Further, a relatively large amount of the material flows. As a result, the molded spline assumes such a shape that a terminal end thereof is larger in tooth thickness than an axial end-side portion thereof, and that the terminal end is substantially increased in O.P.D. in comparison with the other portions. The over pin diameter (O.P.D.) is a value obtained by placing pins of a predetermined diameter on tooth portions of the spline, which face each other at an angle of 180°, respectively, and measuring a maximum spacing dimension between both the pins in a diametral direction of the shaft. Balls of a predetermined diameter may be used instead of the pins (over ball diameter). When the spline mentioned above, whose terminal end is substantially increased in O.P.D., is fitted into a spline hole of a mating member such as an inner wheel of a constant velocity universal joint, the portion with the increased O.P.D. exerts such a force that opens a part of the spline in the spline hole. In consequence, the mating member may decrease in strength.
In addition, when the shaft and the mating member are spline-fitted to each other, the shaft may have a torsional angle in order to restrict the backlash (clearance) of fitted portions. In the case of the spline whose terminal end is substantially increased in O.P.D., however, the torsional angle cannot be optimally set with ease.
The rubber sealing plug disclosed in JP 06-281010 A needs to be press-fitted into a hollow portion with a relatively large force. Therefore, there is caused a problem in that a considerable amount of labor is required in performing a mounting operation.
The elastomer sealing plug disclosed in JP 09-68233 A is designed to simplify a mounting operation through the concomitant use of a stopper insert made of a shape memory alloy. As a result, however, the number of parts is increased.
Both the sealing plugs are available as parts each molded into a desired shape and with a desired dimension. As a result, there is also a problem of higher manufacturing cost.